1. Field
The present disclosure relates to a biodegradable stent and a manufacturing method thereof.
2. Discussion of Related Technology
Biodegradable materials which can be naturally degraded and absorbed in vivo are attracting attention as a new paradigm for biomaterials. The use of biodegradable materials makes it possible to solve problems caused by permanent biomaterials, including stress shielding, accumulation of toxic metal ions, and secondary surgery for removal of inserted materials.
Particularly, synthetic biodegradable polymers such as polylactic acid (PLA), polyglycolic acid (PGA) and polycaprolactone (PCL) have been widely used as a substitute for permanent biomaterials in orthopedic surgical applications. However, despite excellent biocompatibility, biodegradable polymers are limited in use due to low mechanical strength, the possible release of toxic substances after degradation, and a low degradation rate.
Recently, magnesium (Mg) and its alloys have attracted attention as promising biodegradable materials, because they provide excellent advantages, particularly excellent mechanical properties, compared to synthetic biodegradable polymers. Magnesium has an elastic modulus and compressive yield strength similar to those of natural bone and has a fracture toughness greater than that of natural bone. Thus, magnesium and its alloys have a great potential for load-bearing applications. Furthermore, magnesium has excellent biocompatibility, and released magnesium ions are beneficial for the growth of bone tissue, rather than harmful to the human body.
Nevertheless, the use of magnesium in biomedical applications is limited because of its high corrosion rate in an in vivo environment. Magnesium in aqueous solution reacts rapidly to produce byproducts such as Mg2+ ion, hydroxide and hydrogen gas, which may be harmful to the surrounding tissues. This phenomenon is accelerated in chlorine-containing solutions such as body fluid. Because magnesium is degraded very rapidly, it can weaken the stability of biomaterials in vivo and reduce the biocompatibility of magnesium biomaterials. Thus, it is required to guarantee the initial stability of magnesium biomaterials by improving the corrosion resistance of magnesium.
Numerous methods for improving the corrosion resistance of magnesium to allow it to be used in bone implants have been studied, and examples thereof include alloying, mechanical processing and surface modification.
Specifically, magnesium alloys containing zinc (Zn) and manganese (Mn) show much higher corrosion resistance in Hank's solution (Hank's balance salt solution (HBSS)) than does pure magnesium, and binary Mg—Ca alloys have not only significant biocompatibility, but also mechanical and corrosion properties which can be adjusted depending on the calcium content.
Mechanical processing can be used to control the biodegradability of magnesium. For example, by performing hot rolling, the corrosion rate of magnesium alloy AZ31 can be reduced.
Particularly, surface modification of biomaterials is a suitable method for improving all biocompatibility and corrosion properties. It has indeed been used in metal implants such as stainless steel, titanium and its alloys. Because the surface of biomaterials reacts directly with body fluid in vivo and interacts with the surrounding tissues, the surface properties of biomaterials are critical for the performance of the biomaterials. Various surface treatments, including anodizing, electrodeposition, phosphating treatment, ion plating and fluoride conversion coating, have been used to improve the corrosion properties of the magnesium surface.
The results of electrochemical and immersion tests shows that the biodegradation rate of magnesium in chlorine-containing solutions is significantly reduced by the above-described surface treatments. Also, it was found that the Ca—P coating by phosphating treatment can improve the surface cytocompatibility and bioactivity of magnesium. Accordingly, biodegradable magnesium, the corrosion resistance and bioactivity of which are improved by surface coating, can be a preferable biomaterial.
Meanwhile, a stent consisting only of a magnesium layer have high in vivo corrosion rate, which lead to poor biostability and biocompatibility. In an example shown in FIG. 1, a stent is manufactured by forming an additional metal layer such as a titanium layer on a magnesium layer. However, in this example, if cracks occurred in the titanium layer due to corrosion or if crevices occurred in the titanium layer for other reasons, the magnesium layer was corroded more rapidly due to the cracks or crevices.
The foregoing discussion is to provide general background information, and does not constitute an admission of prior art.